Add a Comment VS Reply
If you click on the blue "reply" link at the bottom right of someone's response, you're essentially responding to their post specfically and they will get notified (though everyone else will also get to see your response). If you want to respond to the author of the blog, then go to the very bottom of the document and click on the red "add a comment." The two buttons act in slightly different ways. If someone has unsubscribed from the general comments, for example, replying to their particular response will get their attention, I think. Either way the author of the piece (and everyone else) will see you comment. It just makes it a little more easy to understand who's commenting to whom. If this makes any sense whatsoever.